(1 of )Ohio State defensive back Marshon Lattimore runs the 40-yard dash at the NFL football scouting combine in Indianapolis, Monday, March 6, 2017. (AP Photo/Michael Conroy)

(2 of )FILE --In this Oct. 1, 2016, file photo, Vanderbilt linebacker Zach Cunningham plays against Florida in an NCAA college football game in Nashville, Tenn. Cunningham is making his case for Southeastern Conference defensive player of the year with one remarkable play after another. The Vanderbilt linebacker made the game-saving stop on fourth-and-1 last month and hurdled the line to block a field goal last week at Auburn. (AP Photo/Mark Humphrey, File)

(3 of )Kansas State defensive end Jordan Willis (75) is shown in action during the second half of the Texas Bowl NCAA college football game against Texas A&M, Wednesday, Dec. 28, 2016, in Houston. Kansas State won the game, 33-28. (AP Photo/Eric Christian Smith)

(4 of )Utah Utes running back Joe Williams (28) warms up before an NCAA college football game Saturday, Nov. 26, 2016, in Boulder, Colo. Colorado won 27-22. (AP Photo/David Zalubowski)

Grant Cohn: 49ers still have work to do in offseason — here’s the list

The 49ers’ focus during free agency has been signing veterans on offense. This is smart.

Kyle Shanahan’s system is complicated. Young players take more than half a season to learn it. So, by signing veterans — especially ones who have played for Shanahan and already know his system — the team eliminated the learning curve. The offense should be ready to compete Week 1.

But that doesn’t mean the whole team will be ready. If the Niners are serious about making a playoff run next season — and I believe they are serious — these are the five assignments they still must complete.

Assignment No. 1. Draft a cornerback who can start.

The new defensive coordinator, Robert Saleh, comes from the Pete Carroll tree of coaches. These coaches all run the same system, and it’s simple, unlike Shanahan’s offense.

The philosophy is to use only a few coverage schemes so players don’t have to think much — they can just react. This allows rookies to learn the system quickly, which is another reason the Niners were smart to focus on offense during free agency. The team can find defensive starters in the draft.

First, the 49ers need to find a cornerback. This defense doesn’t work without terrific corners (think Richard Sherman). That’s because the system places the strong safety “in the box” — right in front of the tight end — so the cornerbacks don’t have much help. They’re mostly on their own. They have to be fantastic.

The Niners don’t have fantastic corners. They have one who’s young and talented — Rashard Robinson — and one who’s old and short — Tramaine Brock. He doesn’t fit the new scheme. The team needs to replace him.

I suggest replacing him by drafting Marshon Lattimore from Ohio State in Round 1. He’s the best corner in the draft. He fits any scheme.

Armstrong is a backup — he lasted only two games as a starter last season before he tore a chest muscle. Smith is limited — he can’t cover tight ends or running backs. And Bowman is injured — he tore his Achilles tendon in Week 3. Achilles tendons typically take at least a full year to heal. Bowman may not be ready to play when the season starts.

The Niners have to draft a linebacker with one of their first two picks. I suggest drafting Zach Cunningham from Vanderbilt in Round 2. Right away, he would be the team’s best linebacker — the only one who excels stopping both the run and pass.

Assignment No. 3. Draft a pass rusher.

The Niners have only one who’s good — Aaron Lynch. But, he wasn’t good last season. He failed a drug test and was suspended. When he came back, he was out of shape. Then he sprained his ankle.

The other pass rushers are Ahmad Brooks (old), Eli Harold (bust) and Tank Carradine (even bigger bust). The 49ers won’t make the playoffs with this group.

I suggest they spend their third-round pick on Jordan Willis from TCU. He’s extremely athletic and he recorded 11.5 sacks in 13 games as a senior.

As you may notice, I’m suggesting the 49ers spend their first three draft picks on defense. Remember, last season the Niners had the worst defense in the league and in franchise history. No messing around here. The team can’t afford to spend a high pick on a backup quarterback.

Assignment No. 4. Draft a No. 2 running back.

Time to focus on the offense again.

Shanahan probably envisions a run-first attack and an effective play-action passing game to make things easier for his quarterback, Brian Hoyer. Hoyer isn’t good enough to drop back and chuck the ball all game.

For Shanahan’s vision to become reality, the 49ers need another running back. Carlos Hyde can’t be the only one — he gets injured too often. If he gets injured next season, Hoyer will have to carry the offense, and that would mean no playoffs.

I suggest the Niners spend a fourth-round pick on Joe Williams from Utah. Williams averaged 6.7 yards per carry last season — better than Christian McCaffrey, who averaged 6.3 in the same conference, the Pac-12.

Assignment No. 5. Rest the veterans.

This is the most important assignment.

Shanahan’s offense will fall apart if he overworks the older players he signed this offseason. He needs to go easy on them. Needs to give them days off. Needs to make sure he doesn’t grind them.

Shanahan can’t let this team fade from exhaustion late in the season. To make the playoffs, the Niners will have to win close games in December, and they won’t win those games with worn-out veterans.

If Shanahan keeps them fresh and healthy, I like the Niners’ chances.

Grant Cohn writes sports columns and the “Inside the 49ers” blog for The Press Democrat’s website. You can reach him at grantcohn@gmail.com.